Update Dockerfile to pass env variables

Dockerfile

@@ -12,6 +12,13 @@ RUN npm ci
 # Copy source code
 COPY . .
 
+# Build-time env (Vite inlines these at build time)
+ARG VITE_BACKEND_API_URL
+ARG VITE_ENV=production
+
+ENV VITE_BACKEND_API_URL=${VITE_BACKEND_API_URL}
+ENV VITE_ENV=${VITE_ENV}
+
 # Build the application
 RUN npm run build:prod